1. Field of the Invention
The present invention relates to a battery charger for use with a chargeable battery such as a Ni-Cd battery.
2. Description of the Prior Art
Generally, when a battery is charged, its voltage varies with time as shown by line A in FIG. 1(A). At the beginning of a charging operation, the battery voltage increases relatively rapidly (see line part A1). The battery voltage thereafter gently increases (see line part A2) with time, and the rate of increase is again raised at a time immediately before the battery is fully charged (see line part A3). If the charging operation is still continued after the battery has been fully charged, the battery voltage decreases with time. Thus, the battery voltage will have a peak value at the time when the battery is fully charged. Various techniques have been proposed to utilize this phenomenon for stopping the charging operation, by means of detection of a timing of conversion from the increasing tendency to the decreasing tendency, at the time when the battery is fully charged.
Japanese Laid-Open Patent Publication No. 63-234844 discloses a technique to detect the battery voltage at intervals of a predetermined period and to stop the charging operation when the battery voltage is decreased continuously by a number of times more than a predetermined number, so that the charging operation can be stopped immediately after the battery has been fully charged.
With this technique, however, the charging operation for the battery cannot be successfully performed when the battery is an over-discharged one. When the over-discharged battery is charged, the battery voltage firstly abruptly increases as indicated by line part A5 in FIG. 1(A), and subsequently decreases with time (see line part A6). The battery voltage thereafter increases in the same pattern as the normal battery. According to the technique of detecting merely the timing when the battery voltage is converted from the increasing tendency to the decreasing tendency, the timing of the conversion from line part A5 to line part A6 is detected. Therefore, the charging operation is stopped when the battery is short of charging.
In order to prevent such malfunction, various techniques have been also proposed. Japanese Laid-Open Patent Publication No. 4-58471 discloses a technique in which the determination as to whether the battery voltage has reached the peak value is not performed so long as the battery voltage is less than a predetermined value. Japanese Laid-Open Patent Publication No. 61-288740 discloses a technique in which the determination as to whether the battery voltage has reached the peak value is not performed until the battery voltage begins to stably increase. Japanese Laid-Open Patent Publication No. 63-234844 mentioned above discloses a technique in which the determination as to whether the battery voltage has converted from the increasing tendency to the decreasing tendency is not performed until a predetermined time elapses after starting the charging operation.
With the technique of Japanese Laid-Open Patent Publication No. 4-58471 or the technique in which the determination as to whether the battery voltage has reached the peak value is inhibited as long as the battery voltage is less than the predetermined value, it is difficult to charge batteries having different rating voltages by using a common battery charger. For example, in order to make the battery charger applicable for charging both batteries having rating voltages of 12.0 V and 7.2 V, the predetermined value must be determined to be higher than a peak voltage (V7 in FIG. 1(A)) which may be produced at the beginning of the charging operation of the over-discharged 12.0 V battery. However, if the predetermined value has been determined to be such a higher value, it becomes impossible to determine as to whether the voltage of the 7.2V battery has reached a peak value properly corresponding to a peak voltage (V8 in FIG. 1(A)) which may appear at the fully charged situation. Thus, with the technique in which the determination as to whether the battery voltage has reached the peak value is performed with reference to the predetermined value, batteries having different rating voltages cannot be charged by using a common battery charger.
With the technique of Japanese Laid-Open Patent Publication No. 61-288740, or the technique in which the determination as to whether the battery voltage has reached the peak value is not performed until the battery voltage begins to stably increase, and with the technique of Japanese Laid-Open Patent Publication No. 63-234844 in which the determination as to whether the battery voltage has reached the maximum value is not performed until the predetermined time elapses after starting the charging operation, the determination may be started from the state corresponding to line part A2 in FIG. 1(A). Here, the battery voltage during the period of line part A2 stably increases though gently, and therefore, the determination for the peak value is expected to be properly performed. However, in a practical charging operation at a working place, a high-power-output motor may be operated to be started and stopped in some cases, so that the power source voltage may be varied with such operation of the motor. Therefore, during the period of line part A2, the battery voltage may be temporarily dropped under the influence of the variation of the power source voltage. Such temporary drop of the voltage may be also caused when the contacting condition between the battery and the battery charger is varied. When this occurs, the charging operation is stopped before the battery is fully charged, so that the problem of shortness of charge may be caused in spite of applications of these techniques. Particularly in case of application of the technique of Japanese Laid-Open Patent Publication No. 63-234844, it is necessary to prevent over-charge of the battery when the battery already charged is recharged, and therefore, the predetermined time cannot be determined so long. Thus, the period indicated by line part A2 will become the subject of the determination in this technique.